Why do chromates give corrosion resistance to aluminum?

I am a first year student, studying physical sciences at the University of Toronto. I have been asked to write a report as if I were an aircraft engineer that was defending the use of primers containing dichromate pigments to inhibit corrosion of aluminum. I need to know what hazards the metal needs to be protected from and why Cr(VI) is so good at protecting Aluminum from corrosion.

Thank You

There were several papers presented on this subject at September Cadmium, Chromium, and Nickel Alternatives Information Exchange, put on by the National Defense Center for Environmental Excellence (NDCEE). Derek DeMaree from the Army Materials Laboratory gave the best presentation I've seen on why chromate works. I suggest you contact Derek directly, but you can also contact NDCEE for a copy of his presentation.

Hi. Although Jim's answer is a good one, and appreciated, the defense industry has largely moved away from hexavalent chromates for aluminum and now considers certain trivalent chromates equivalent in corrosion protection.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey